Carbonated Silicate Melt Research Articles

Linking deep CO2 outgassing to cratonic destruction.

Outgassing of carbon dioxide from the Earth's interior regulates the surface climate through deep time. Here we examine the role of cratonic destruction in mantle CO2 outgassing via collating and presenting new data for Paleozoic kimberlites, Mesozoic basaltic rocks and their mantle xenoliths from the eastern North China Craton (NCC), which underwent extensive destruction in the early Cretaceous. High Ca/Al and low Ti/Eu and δ26Mg are widely observed in lamprophyres and mantle xenoliths, which demonstrates that the cratonic lithospheric mantle (CLM) was pervasively metasomatized by recycled carbonates. Raman analysis of bubble-bearing melt inclusions shows that redox melting of the C-rich CLM produced carbonated silicate melts with high CO2 content. The enormous quantities of CO2 in these magmas, together with substantial CO2 degassing from the carbonated melt–CLM reaction and crustal heating, indicate that destruction of the eastern NCC resulted in rapid and extensive mantle CO2 emission, which partly contributed to the early Cretaceous greenhouse climate episode.

Post-spreading volcanism triggered by CO2 along the South China Sea fossil spreading axis

The post-spreading seamount chain formed along the fossil ridge of the South China Sea (SCS) consists of carbonated silicate melts and alkaline basalts. These temporally and spatially linked samples give an intriguing case where CO2-rich magmas and associated basaltic rocks occurred upon the ultra-thin oceanic lithosphere, and thus they could carry robust information on the generation of alkaline magma via melting of carbonated mantle source. Here we present magnesium isotopic data (δ26Mg), and elemental and radiogenic isotope data for the lavas from the on-axis SCS seamount chain. Both the early- and late-stage samples classified based on their formation ages exhibit remarkable δ26Mg variations (−0.53‰ to −0.11‰ and − 0.38‰ to −0.23‰, respectively). The low-δ26Mg signatures found in two groups of the SCS samples indicate a recycled origin for the source carbon inventory. However, the low-δ26Mg endmember of the early-stage samples is characterized by low SiO2 and enrichment in rare earth elements (REEs), whereas the late-stage samples display a low-δ26Mg endmember that produced high-SiO2 and REE-depleted melts. The Mg-Nd isotope systematics suggest that the early-stage lavas were yielded by CO2-enhanced melting of sediment-rich oceanic crust (εNd = 4.2), whereas the late-stage lavas have sourced the residual and carbon-deficient oceanic crust (εNd = 8.7). Our model demonstrates that the melting of carbonated component can produce compositionally distinct carbonatitic and silicate melts in sequence due to different solidus temperatures. Owe to the relatively thin lithosphere, these two types of melt could individually appear along the fossil ridge of the SCS.

Carbon flux and alkaline volcanism: Evidence from carbonatite-like carbonate minerals in trachytes, Ulleung Island, South Korea

Abstract Carbon flux metasomatism in the subduction environment is an important process, but it remains poorly understood. The paucity of exposed lower crust and upper mantle rocks in continental arcs renders xenoliths a major target for studying the slab-derived carbon cycle. This study of the carbonate phases in volcanic rocks from three drill cores in Ulleung Island, South Korea, sheds light on the interaction of carbon flux in the upper mantle and lower crust in a back-arc setting. The volcanic rocks from Ulleung Island range in composition from trachybasalt to trachyte and contain abundant euhedral pseudomorphic carbonate grains, ulvöspinel-hosted and biotite-hosted carbonate-silicate melt inclusions, and irregular carbonate globules. Integrated petrographic and geochemical studies of a variety of phenocrysts, carbonate phases, and carbonate-silicate inclusions in biotite and ulvöspinel indicate that recharging of carbon flux affected magma evolution. Carbon and oxygen isotopes of the pseudomorphic carbonate grains overlap with mantle values, indicating a carbonatite-like origin of the carbonate phases. The (MgO, FeO, CaO)-rich silicates in ulvöspinel-hosted silicate inclusions and pseudomorphic carbonate grains likely represent a primary melt, which formed from the partial melting of carbonated eclogite of the subducted slab within the mantle wedge beneath Ulleung Island. A petrogenetic model is proposed to illustrate that the crystal mush in the magma chamber was intruded by carbonate-rich liquids and caused alteration of cumulate crystals to generate the euhedral pseudomorphic carbonate grains. The extrusive magma captured those pseudomorphic grains and erupted to form the trachybasalt-trachyte units. The observed carbonate phases and their geochemical characteristics indicate that carbon flux metasomatism played a fundamental role in this back-arc magmatism.

Petrogenesis of silica-rich carbonatites from continental rift settings: a missing link between carbonatites and carbonated silicate melts?

Petrogenesis of silica-rich carbonatites from continental rift settings: a missing link between carbonatites and carbonated silicate melts?

Pyroxenite as a Product of Mafic-Carbonate Melt Interaction (Tazheran Massif, West Baikal Area, Russia)

Pyroxenite and nepheline-pyroxene rocks coexist with dolomite-bearing calcite marbles in Tazheran Massif in the area of Lake Baikal, Siberia, Russia. Pyroxenites occur in a continuous elongate zone between marbles and beerbachites (metamorphosed gabbro dolerites) and in 5 cm to 20 m fragments among the marbles. Pyroxene in pyroxenite is rich in calcium and alumina (5–12 wt% Al2O3) and has a fassaite composition. The Tazheran pyroxenite may originate from a mafic subvolcanic source indicated by the presence of remnant dolerite found in one pyroxenite body. This origin can be explained in terms of interaction between mafic and crust-derived carbonatitic melts, judging by the mineralogy of pyroxenite bodies and their geological relations with marbles. According to this model, the intrusion of mantle mafic melts into thick lower crust saturated with fluids caused partial melting of silicate-carbonate material and produced carbonate and carbonate-silicate melts. The fassaite-bearing pyroxenite crystallized from a silicate-carbonate melt mixture which was produced by roughly synchronous injections of mafic, pyroxenitic, and carbonate melt batches. The ascending hydrous carbonate melts entrained fragments of pyroxenite that crystallized previously at a temperature exceeding the crystallization point of carbonates. Subsequently, while the whole magmatic system was cooling down, pyroxenite became metasomatized by circulating fluids, which led to the formation of assemblages with garnet, melilite, and scapolite.

Partitioning of chalcophile and highly siderophile elements (HSEs) between sulfide and carbonated melts – Implications for HSE systematics of kimberlites, carbonatites, and melt metasomatized mantle domains

Highly Siderophile Elements (HSEs; Os, Ru, Ir, Rh, Pt, Pd, Au and Re) combined with their isotopic systematics (Re-Os and Pt-Os) are powerful tools for tracking evolution and genesis of mantle derived magmas. Given sulfides (accessory sulfide minerals and/or molten sulfides) are the primary hosts of HSEs in the mantle and low-degree carbonated melts are extracted from large portions of mantle volume, partitioning of HSEs between sulfide and carbonated melt might play a critical role in distributing HSEs between the mantle and crustal reservoirs. Although, partitioning of HSEs and chalcophile elements between sulfide melt and silicate melt has been previously studied, partitioning of these elements between sulfide melt and carbonated melts has not received much attention. Here we use high P-T experiments to determine the partitioning of HSEs and chalcophile elements (Ni, Co, Mo, Os, Ru, Pd, Pt and Re) between (i) sulfide melt and carbonated silicate melt (CO2 ~ 17 wt.%) and (ii) sulfide melt and carbonatitic melt (CO2 ~ >30 wt.%) at a pressure (P) of 3 GPa and temperatures (T) of 1300–1600 °C in graphite capsules. All experiments produced quenched Fe-sulfide melt blobs + carbonated silicate melt matrix. Concentrations of major elements were measured using electron microprobe, and HSEs and chalcophile elements were measured using LA-ICP-MS. We find that all the elements measured are compatible in the sulfide melt to varying degrees and their Dsulfide/carb. melt sequence is Mo < Co < Ni < Re < Pt ≤ Pd < Ru ≤ Os varying from around 10 for Mo to 105 for Os. Comparing the Dsulfide-carb. melt with Dsulfide-silicate from previous studies, we show that the partition coefficients of HSEs between sulfide and carbonated melts are lower than the partition coefficients of these elements between sulfide and silicate melts, indicating greater mobilization of these elements in carbonatites and carbonated silicate melts. Calculating bulk D (D¯) for carbonated peridotite using our experimentally measured D values, we model the HSE contents of mantle derived low-degree partial melts using an aggregate fractional melting equation and compare the primitive mantle normalized HSE patterns of our model with natural kimberlites, carbonatites, ocean island basalts, and alkaline basalts. We also calculate proportions of sub-lithospheric continental mantle (SCLM) xenolith detritus in the natural kimberlite and carbonatite samples from Karelian, Kaapvaal, Canadian shield and North China craton by using mass balance calculations based on Ru concentration in the primary carbonated melt and the SCLM xenoliths. Our calculations show that detritus proportion in natural kimberlites are 2–28% for Karelian, 7–28% for Kaapvaal, and 6–16% for Canadian shield, which are in agreement with previous studies using various other proxies. We also show that the extent of Re/Os fractionation is less for events of carbonate melt metasomatism as compared to similar events of basaltic melt metasomatism.

Nephelinites in eastern China originating from the mantle transition zone

Nephelinites, a highly alkaline volcanic rock type, have widely been thought to be formed in a shallow lithosphere previously impregnated with phlogopite and/or amphibole. Here we argue that nephelinites start melting at much greater depths than previously thought. Taking four Cenozoic nephelinitic volcanoes in eastern China as specific examples, we show that nephelinites have exceptionally low K/U and Ba/Th ratios, similar to most nephelinites worldwide, but their Nd-Sr-Hf isotopic compositions are inconsistent with a local lithospheric origin. They also have low, marine‑carbonate-like δ26Mg. We explain these features by partial melting of recycled, carbonate-bearing sediments in the mantle transition zone (MTZ; 410–660 km), where the high-pressure mineral liebermannite (KAlSi3O8, previously known as K-hollandite) is stable as a residual phase, preferentially retaining K, Ba, Rb and Pb relative to Th, U and light REE, thus releasing liquids with the characteristic trace-element patterns. Group-I kimberlites and oceanic HIMU basalts globally share the same peculiar geochemical anomalies characterizing the nephelinites, suggesting they may all have their roots in the MTZ.The nephelinites show correlations for Mg-Sr isotopes, 143Nd/144Nd-MgO, and δ26Mg-Ba/Th ratios, suggesting binary mixing between melts. We therefore invoke a two-stage process to generate these nephelinites. In the first stage, an initial carbonated silicate melt is formed by melting of carbonate-bearing sediments in the MTZ. Subsequently, during the ascent, the initial melt reacts with the surrounding peridotite, thereby generating a reacted melt, which ultimately mixes with the initial, unreacted carbonated silicate melt. Accordingly, Cenozoic nephelinites in eastern China can be regarded as evolved melts from the MTZ.

Polygenic Nature of Olivines from the Ultramafic Lamprophyres of the Terina Complex (Chadobets Upland, Siberian Platform) Based on Trace Element Composition, Crystalline, and Melt Inclusion Data

Olivine from the deep mantle-derived rocks, such as ultramafic lamprophyres, carries important information about the composition of the mantle source, the processes of mantle metasomatism, the origin of specific silicate-carbonate melts, as well as the composition and mechanisms of crystallization of these rocks. Textures and compositions of olivine from the carbonate-rich ultramafic lamprophyres (aillikites) of the Terina complex, along with their mineral and melt inclusions, exposed that olivines have different sources. Two populations of olivines were considered: macrocrysts (&gt;1 mm) and groundmass olivines (&lt;1 mm). Groundmass olivines are phenocrysts and characterized by weak variations in Mg# (84–86.5), a sharp increase in Ca and Ti contents, and a decrease in Ni and Cr from core to rim. They have higher concentrations of Li, Cu, Ti, and Na compared to macrocrysts. Among the macrocrysts, the following populations are observed: (1) high-Mg olivines (Mg# 89–91) with high Ni and low Ti contents, which are interpreted as xenocrysts from the slightly depleted lherzolite mantle; (2) high-Ca olivines (Mg# 84–88, CaO 0.13–0.21 wt %), which have patterns similar to groundmass olivines and are interpreted as cumulates of early portions of aillikite melt; (3) macrocrysts with wide variations in Mg# (73–88), low CaO contents (0.04–0.11 wt %), and positive slope in Ca vs. Al and negative slope in Ca vs. Mn, which are interpreted as disintegrated megacrysts from the Cr-poor megacryst suite. The megacryst suite could have been formed in the pre-trap period during the melting of the metasomatized subcontinental lithospheric mantle (SCLM). The aillikite melt evolution is traced by secondary melt inclusions in olivine macrocrysts: early phlogopite-diopside-calcite-apatite association, containing Ti-magnetite and ilmenite, is followed by an association with magnetite and sulfides (pyrrhotite and pentlandite); finally, at a late stage, inclusions with a predominance of Ca-Na-carbonates and sulfates and enriched in U, Th, Y, REEs, Sr, and Ba were captured.

Metamorphic microdiamond formation is controlled by water activity, phase transitions and temperature

Metamorphic diamonds hosted by major and accessory phases in ultrahigh-pressure (UHP) metamorphic terranes represent important indicators of deep subduction and exhumation of continental crust at convergent plate boundaries. However, their nucleation and growth mechanisms are not well understood due to their small size and diversity. The Bohemian microdiamond samples represent a unique occurrence of monocrystalline octahedral and polycrystalline cubo-octahedral microdiamonds in two different metasedimentary rock types. By combining new and published data on microdiamonds (morphology, resorption, associated phases, carbon isotope composition) with P–T constraints from their host rocks, we demonstrate that the peak P–T conditions for the diamond-bearing UHP rocks cluster along water activity-related phase transitions that determine the microdiamond features. With increasing temperature, the diamond-forming medium changes from aqueous fluid to hydrous melt, and diamond morphology evolves from cubo-octahedral to octahedral. The latter is restricted to the UHP-UHT rocks exceeding 1100 °C, which is above the incongruent melting of phengite, where microdiamonds nucleate along a prograde P–T path in silicate-carbonate hydrous melt. The observed effect of temperature on diamond morphology supports experimental data on diamond growth and can be used for examining growth conditions of cratonic diamonds from kimberlites, which are dominated by octahedra and their resorbed forms.

Global receiver function observations of the X-discontinuity reveal recycled basalt beneath hotspots

The distribution of chemical heterogeneity beneath hotspots provides important constraints on the source of magmatism and mantle convection. Chemical heterogeneities in the upper mantle produce discontinuities that can be interrogated seismically. One such discontinuity, the X-discontinuity (230-350 km depth), is observed intermittently across the globe, with multiple possible causal mechanisms. However, to better understand the cause of the X-discontinuity and its relationship to mantle upwellings, we require global short wavelength observations, previously unresolved by reflected phases with broad upper mantle Fresnel zones. Discontinuities resulting from seismically observable impedance contrasts can be targeted using high frequency P wave receiver functions (RFs) possessing short wavelength lateral sensitivity (∼100 km) to structures almost directly beneath the station. Thus, below well-instrumented hotspots we can investigate the properties of the X-discontinuity and constrain the causal mechanisms in these locations.Using P-to-s converted teleseismic phases between 30-90∘, we stack RFs in the depth and time-slowness domains at 28 hotspots and 6 cratons globally. This reveals 15 robust X-discontinuity observations beneath hotspots between 244-344 km depth. A further 10 potential observations are present beneath hotspots, and 6 null observations beneath cratons. Two causal mechanisms remain possible at the elevated mantle temperatures expected beneath hotspots. The coesite-stishovite phase transition occurs in eclogite and/or carbonated silicate melt may form under certain conditions. For either mechanism to provide seismically observable impedance contrasts, the upper mantle must be locally enriched in basalt. Assessing the X-discontinuity global distribution reveals a correlation with the Large Low Velocity Provinces, suggesting that mantle plumes may entrain recycled basalt from the lowermost mantle.

Low hydrogen concentrations in Dharwar cratonic lithosphere inferred from peridotites, Wajrakarur kimberlites field: Implications for mantle viscosity and carbonated silicate melt metasomatism

Hydrogen as an atomic impurity in mantle minerals is recurrently proposed as a key element impacting significantly on many mantle properties and processes such as melting temperature and mechanical strength. Nevertheless, interpretation based on the natural samples remains weak as we do not have yet a robust world-wild database for hydrogen concentrations in mantle minerals and rocks. Here, we report the first hydrogen concentrations in nominally anhydrous minerals from a rare selection of ultramafic rocks and minerals embedded in Mesoproterozoic Wajrakarur kimberlites (Eastern Dharwar craton, India). Based on key chemical elements, we demonstrate that olivine, pyroxenes and garnet from the Dharwar craton are of mantle origin. We quantify the hydrogen concentrations using Fourier transform infrared spectroscopy (FTIR) and mineral-specific FTIR calibrations. Calculated hydrogen concentrations are, in average, 18 ppm wt H2O in olivine, 70 ppm wt H2O in orthopyroxene and 207 ppm wt H2O in clinopyroxene. Garnet has highly variable hydrogen concentration ranging from 0 to 258 ppm wt H2O, probably influenced by nano-scale inclusions. The average of clean garnet spectra yields 14.5 ppm wt H2O. The reconstructed hydrogen bulk concentrations of Dharwar peridotites yields 40-8+10 ppm wt H2O. This value is two to five times lower than the estimated hydrogen concentration in the lithospheric mantle, and agree well with the lower range of hydrogen bulk concentration from the current data base for the upper mantle minerals transported by kimberlites from other cratons (e.g., South Africa, Siberia). The low hydrogen concentration in mantle minerals, together with petrological and geochemical evidence of carbonated silicate melt metasomatism in Dharwar cratonic lithospheric mantle, suggest that these xenoliths are possibly related to proto-kimberlite melts with low water activity prior to being transported to the surface by the Mesoproterozoic Wajrakarur kimberlites. These observations, valid to a depth of ~165-km, suggest that cratonic lithosphere beneath the Dharwar craton may not be particularly indicative of an abnormal hydrogen-rich southern Indian lithosphere in the late Archean and that hydroxylic weakening in olivine would induced a negligible effect on the mantle viscosity of Indian subcontinent.

Evidence for the essential role of CO2 in the volcanism of the waning Caroline mantle plume

Deep-rooted mantle plumes provide a link for carbon between Earth’s interior and its surface. Low-degree partial melting of CO2-bearing mantle is predicted to produce carbonated melts in the deep mantle, but carbonated melts are rarely found in modern oceanic settings, and the role of primary carbonated silicate melts in mantle-plume volcanism remains unclear. Because low-degree mantle melting tends to generate CO2-rich melts, we studied a suite of volcanic rocks from Pohnpei Island formed by the waning Caroline mantle plume based on whole-rock major- and trace-element, Sr–Nd–Pb–Hf isotopic, and high-precision in situ mineral compositions. We demonstrate that the mineralogy and geochemistry of these volcanic rocks are consistent with those of high-MgO primary carbonated silicate melts, e.g., they have the most SiO2-depleted and CaO-enriched compositions reported for ocean-island basalts, they are enriched in rare-earth elements and have negative anomalies of high-field-strength-elements. The Sr–Nd–Pb–Hf isotopic compositions of these volcanic rocks exhibit mixing between the Pacific-type depleted mantle and an enriched-mantle II (EM2)-like source. We found that primary carbonated silicate melts were modified to become typical ocean-island alkali basalts through reactive evolution and CO2 degassing during magma ascent in the lithosphere, and also identified a closed-system stage of evolution after these melts reached equilibrium with the lithospheric mantle. We suggest that low-degree melting of a waning mantle plume facilitates carbonated-silicate-melt volcanism, with the carbonated silicate melts being converted to typical alkali ocean-island basalt in the overlying thick lithospheric mantle. The anomalously low olivine Ca and Mn contents and the Ca partition coefficient between olivine and melt suggest that the primary magmas had a CO2 content of 11–15 wt.%. These volcanic rocks also contain Hawaii-like high-Ni olivines, which we interpret as reflecting a pyroxenite-rich and olivine-poor mantle source. The proposed reactive evolution of carbonated melts in the lithospheric mantle tends to cause loss of CO2 in the deep mantle and regulates the fate of carbonated melts, which may explain the widespread occurrence of alkali basalts and the rarity of carbonated melts in intra-oceanic-plate settings.

Rare earth elements deposits in China: Spatio-temporal distribution and ore-forming processes

<p indent=0mm>China dominates the world REE (rare earth elements) resources, 98% of which are from carbonatite-related REE deposits. Extensive studies of this type of REE deposits are available, but it is still poorly understood how carbonatite magmas were generated and how REEs were transported and precipitated in such magmas. On the basis of previous studies, this paper synthesizes the spatio-temporal distribution of this type of REEs deposits in China and their major features, including characteristics of ore-forming fluids, and discusses existing genetic models regarding the origin of causative magma, and mechanics of REE enrichment, transportation and precipitation. In China, carbonatite-related REE deposits occur in the craton margin environments with four major REE metallogenic belts: the Mesoproterozoic Bayan Obo-Langshan belt (e.g., the Bayan Obo deposit), the early Mesozoic eastern Qinling-Dabie belt (e.g., the Miaoya deposit), the late Mesozoic Zibo-Laiwu-Weishan belt (e.g., the Chishan deposit), and the Cenozoic Mianning-Dechang belt (e.g., the Maoniuping deposit). In these belts, REE-hosting carbonatites were emplaced in continental rift zones or along large-scale strike-slip faults at the Precambrian craton margins. The causative magmas were originated from partial melting of enriched lithospheric mantle sources, which may have undergone metasomatism by subducted slab-related fluids/melts, and may have also asthenospheric material involvement. These magmas may have formed through melt immiscibility and fractional crystallization from primary carbonated silicate melt. The melt immiscibility may include unmixing between alkaline silicate and carbonate melts, or multi-phase melt immiscibility of carbonate melt, mafic or ultramafic melt, phosphomagnesite melt, and alkaline silicate melt. The immiscibility and/or crystal fractionation of mafic mineral can lead to the formation of carbonate melts rich in REEs. The newly discovered lithofacies zonation at Bayan Obo, with a biotitite outer zone, implies a magmatic evolution of a silica-rich parent magma. The low REE concentrations and LREE/HREE ratios of biotitite indicate possible LREE enrichment in the evolved carbonatite magma by biotite fractional crystallization. The crystallization of carbonate minerals during the upwelling and emplacement of carbonatite magmas, may further enrich late-stage carbonatite melts and carbonatite fluids in LREE. This process can better explain the REE mineralization dominantly in the late-stage of carbonatite magmatism or post-magmatic fluid stage. The melt, melt-fluid and fluid inclusion results for Bayan Obo, Maoniuping and Chishan indicate SO₄²⁻ and CO₂-rich carbonatite fluids. The microthermometry and SEM/EDS results for primary melt-fluid inclusions hosted in fluorite from Maoniuping indicate that carbonatite fluids were extremely rich in SO₄²⁻, and underwent an unmixing process between sulfate melt and aqueous fluid. Both inclusion and experimental results indicate that the sulfate has a very high solubility in SiO₂-bearing aqueous fluids, inconsistent with the SiO₂-free system. The geochemical thermodynamic results also confirm the SO₄²⁻ -dominated agent for REE transportation. The unmixing process between sulfate melts and aqueous fluids would lead to extensive decrease of SO₄²⁻ in the fluid, and bulk REE precipitation. The rapid evolution of carbonatite fluids prevents long distance migration of carbonatite fluids, resulting in all REE mineralization within, or proximal to the carbonatite bodies. The strong mineralization would occur in the central zone or the shallow level and then would become weaker outwards and downwards of carbonatite bodies.

Lithospheric modification by carbonatitic to alkaline melts and deep carbon cycle: Insights from peridotite xenoliths of eastern China

Carbonates in subducting oceanic slabs can survive beyond slab dehydration and be transferred into the deep mantle. Such deep carbon cycling plays a critical role in generating carbonatitic to alkaline melts. However, whether and how this process has influenced the lithospheric mantle still remains enigmatic. To address these issues, here we provide a detailed petrographic, in-situ chemical and Sr isotopic study on two mantle xenoliths (a wehrlite and a melt pocket-bearing peridotite) entrained by the Changle Miocene basalts from the eastern China. The Changle wehrlite contains carbonate melt inclusions and apatites and is merely enriched in clinopyroxene relative to the Changle lherzolites. The clinopyroxenes are characterized by high (La/Yb)N (4.7–41) and low Ti/Eu (873–2292) ratios and equilibrated with carbonated silicate melt-like compositions. These petrographic and chemical features indicate that the wehrlite was formed by reaction between peridotite and carbonated silicate melts. On the other hand, the Changle melt pocket-bearing peridotite is suggested to have been produced by in-situ melting/breakdown of amphiboles of an amphibole-rich dunite. Low olivine Fo (~89), presence of amphiboles with high (La/Yb)N (~50) and low Ti/Eu (~1070) ratios suggest that such amphibole-rich dunite would have been formed by reaction of peridotite with hydrous alkaline basaltic melts from a carbonated mantle. Our data, combined with previously reported data of the Changle lherzolite xenoliths, unravel a series of mantle metasomatisms by carbonatitic to alkaline melts from carbonated mantle sources. The consistently high 87Sr/86Sr ratios (up to 0.7036) of the clinopyroxenes in both the wehrlites and lherzolites indicate the carbonate components in the mantle sources were derived from the stagnant Pacific slab within the Mantle Transition Zone. This study provides a fresh perspective on the role of deep carbon cycling from subducted oceanic slabs in chemical modification of intracontinental lithospheric mantle through reaction with different types of melts.

Plume activity and carbonated silicate melt metasomatism in Dharwar cratonic lithosphere: Evidence from peridotite xenoliths in Wajrakarur kimberlites

We report petrography, mineralogy, major- and trace-element compositions of a rare selection of spinel- and garnet-bearing peridotite xenoliths and single crystals separated from peridotites hosted in the Mesoproterozoic Wajrakarur kimberlites from the Eastern Dharwar craton (EDC), India. These ultramafic xenoliths consist of olivine (modal 74–82 vol. %) with Fo92–93, clinopyroxene, orthopyroxene, spinel, garnet, and/or ilmenite. Calculated equilibrium pressure and temperature conditions are 2.5–5.0 GPa and 710–1179 °C for these peridotites, which suggests residence depths >160 km near the base of the Dharwar cratonic lithospheric mantle. Garnet in these ultramafic rocks [with Mg# = molar (Mg/(Mg + Fetotal) × 100 of 80–88] displays either “sinuous” LREE-enriched patterns with depletion in Gd and Er for harzburgites or “normal” LREE-depleted, HREE-enriched patterns for lherzolites. Two groups of clinopyroxenes (group-I and group-II) were also observed with high LREE (LaN > 10) and low LREE (LaN < 10), respectively. The Yb vs. Zr contents in peridotitic garnet are characterised by two distinct trends: one with low-T metasomatism for harzburgitic garnets and the other with high-T metasomatism for lherzolitic garnet, which suggests metasomatism from fluids circulating with in the continental lithospheric mantle, resulting in refertilization from harzburgite to lherzolite. REE concentrations of hypothetical melts calculated to be in equilibrium with peridotitic garnet resemble small volume carbonated silicate melts closely similar to natural kimberlite and lamproites from the Dharwar craton. Modeling suggest that olivine-rich peridotites of the Dharwar craton are the residues of after ~35–50% of melt extraction at ~1500–1540 °C and ≤ 2.0 GPa (≤60 km depth). Residues of these high-Mg# (~92–93) peridotites were generated by shallow and a hot plume melting environment. These processes formed a thick depleted mantle residue of primordial cratonic nucleus of Eastern and Western Dharwar. Later, these two blocks collided and were amalgamated, which further thickened the lithospheric mantle beneath the Dharwar craton. The peridotites later were subjected to metasomatic modification, possibly related to proto-kimberlite activity prior to being transported to the surface by the Mesoproterozoic Wajrakarur kimberlites.

Late Cenozoic Uguumur and Bod-Uul Volcanic Centers in Northern Mongolia: Mineralogy, Geochemistry, and Magma Sources

The paper presents new data on mineralogy, geochemistry, and Sr-Nd-Pb isotope systematics of Late Cenozoic eruption products of Uguumur and Bod-Uul volcanoes in the Tesiingol field of Northern Mongolia, with implications for the magma generation conditions, magma sources, and geodynamic causes of volcanism. The lavas and pyroclastics of the two volcanic centers are composed of basanite, phonotephrite, basaltic trachyandesite, and trachyandesite, which enclose spinel and garnet peridotite and garnet-bearing pyroxenite xenoliths; megacrysts of Na-sanidine, Ca-Na pyroxene, ilmenite, and almandine-grossular-pyrope garnets; and carbonate phases. The rocks are enriched in LILE and HFSE, show strongly fractioned REE spectra, and are relatively depleted in U and Th. The low contents of U and Th in Late Cenozoic volcanics from Northern and Central Mongolia represent the composition of a magma source. The presence of carbonate phases in subliquidus minerals and mantle rocks indicates that carbon-bearing fluids were important agents in metasomatism of subcontinental lithospheric mantle. The silicate-carbonate melts were apparently released from eclogitizied slabs during the Paleo-Asian and Mongol-Okhotsk subduction. The parent alkali-basaltic magma may be derived as a result from partial melting of Grt-bearing pyroxenite or eclogite-like material or carobantized peridotite. The sources of alkali-basaltic magmas from the Northern and Central Mongolia plot different isotope trends corresponding to two different provinces. The isotope signatures of megacrysts are similar to those of studied volcanic centers rocks. The P-T conditions inferred for the crystallization of pyroxene and garnet megacrysts correspond to a depth range from the Grt-Sp phase transition to the lower crust. Late Cenozoic volcanism in Northern and Central Mongolia may be a response to stress propagation and gravity instability in the mantle associated with the India-Asia collision.

Eclogites of the North Atlantic Craton: insights from the Chidliak eclogite xenoliths (S. Baffin Island, Canada)

The 156–138 Ma Chidliak kimberlites on the Eastern Hall peninsula (EHP) of Baffin Island entrained mantle xenoliths interpreted to have been a part of the Archean North Atlantic Craton (NAC) lithospheric mantle. We studied 19 Chidliak eclogite xenoliths that comprise 10 bimineralic, 5 rutile-bearing, 3 orthopyroxene-bearing and 1 kyanite-bearing eclogites. We report major and trace element compositions of the minerals, calculated bulk compositions, pressures and temperatures of the rock formation and model melt extraction from viable protoliths. The eclogite samples are classified into three groups of HREE-enriched, LREE-depleted and metasomatized based on their reconstructed whole-rock REE patterns. PT parameters of the eclogites were calculated by projecting garnet–clinopyroxene temperatures onto the local P–T arrays for 65 Chidliak peridotite xenoliths. All Chidliak eclogites are equilibrated in the diamond P–T field and cluster in two groups, low-temperature (n = 5, 840–990 °C at 4.1–5.0 GPa) and high-temperature (n = 11, T > 1320 °C at P > 7.0 GPa). The reconstructed Mg-rich major element bulk compositions and trace elements patterns are similar to Archean basalts from the North Atlantic and Superior cratons and the oceanic gabbros. The LREE-depleted Chidliak eclogites could be residues after 15–55% partial melting of Archean basalt at the eclogite facies of metamorphism that led to extraction of a tonalite–trondhjemite–granodiorite melt from the EHP. The HREE-depleted eclogites may have experienced a lower degree ( 95%. The former are more magnesian, less ferrous and calcic, contain more magnesian and less calcic garnets, and lower proportions of group C eclogites. The contrast may relate to the stronger NAC metasomatism by silicate–carbonate melt observed in Chidliak peridotitic mantle, or to the different formation ages of the eclogites beneath the two cratons.

Causes and Consequences of Wehrlitization Beneath a Trans‐Lithospheric Fault: Evidence From Mesozoic Basalt‐Borne Wehrlite Xenoliths From the Tan‐Lu Fault Belt, North China Craton

AbstractTrans‐lithospheric faults represent weak zones where some of the most intense interaction between the lithospheric mantle and melts occurs. We carried out detailed petrological observations and mineral chemical analyses of feldspar‐bearing wehrlites entrained in the Mesozoic basalts from Liaoyuan, northeastern North China Craton (NCC) to obtain new insights into the nature and consequences of wehrlitization. Wehrlites (n = 12) show textural evidence for replacement of orthopyroxene by clinopyroxene, combined with low Ti/Eu (692–4,425) and variable, though generally high, Ca/Al, (La/Yb)N, and Zr/Hf in clinopyroxenes. This is ascribed to interaction with a silica‐undersaturated, carbonated silicate melt. Feldspar with variable K2O (0.17–9.84 wt.%) and CaO (0.03–12.8 wt.%) and some clinopyroxene likely formed by decomposition of amphibole and mica during heating prior to entrainment. Integrated with data from peridotite xenoliths in Cenozoic basalts along the Tan‐Lu fault belt (TLFB), these observations indicate deep volatile‐rich melt metasomatism. This may have occurred during a known Late Cretaceous to Early Tertiary extensional stage and was precursory to basaltic magmatism in the north‐south direction along the TLFB. We suggest that wehrlitization beneath the TLFB—and elsewhere—monitors the passage and mobility of carbonated melts linked to lithospheric thinning up to decratonization. Upward transport of the wehrlitizing carbonated melts through the massive TLFB and ultimate degassing of CO2 at the surface during its extensional stage may have contributed to the mid‐Cretaceous greenhouse climate.

The impact on mantle olivine resulting from carbonated silicate melt interaction

Interactions between carbonated ultramafic silicate magmas and the continental lithospheric mantle results in the formation of dunite—a ubiquitous xenolith type in kimberlites and aillikites. However, whether this process dominantly occurs in the mantle source region or by subsequent interactions between lithospheric mantle fragments and transporting silica-undersaturated magmas during ascent remains debated. Aillikite magmas, which are derived from the fusion of carbonate-phlogopite metasomes under diamond-stability field upper mantle conditions, have a mineralogically more complex source than kimberlites, providing an opportunity to more fully constrain the origin of dunite xenoliths in such deeply sourced carbonated silicate magmas. Here we present a major and trace element study of olivine occurring in xenoliths and as phenocrysts in an aillikite dike located on the southern Superior Craton. We show that olivine within the dunite microxenoliths exhibits extreme enrichment in Al, Cr, Na, and V when compared to equivalent xenoliths carried by kimberlites. We interpret these results as evidence for the presence of carbonate-phlogopite metasomes left residual in the cratonic mantle source during aillikite magma formation. Our results are inconsistent with models of dunite formation through orthopyroxene dissolution upon kimberlite/aillikite magma ascent, supporting an origin for such dunites that is more closely linked to primary melt generation at the base of relatively thick continental lithosphere. Our work demonstrates that it is possible to constrain the precursor composition of cratonic mantle dunite at depth, thereby facilitating the further exploration of how carbonated silicate magmas modify and weaken continental lithospheric roots.

Experimental study of the system peridotite–basalt–fluid: phase relations at supra- and sepercritical P-T parameters

To obtain new data on the phase relationships in the fluid-containing upper mantle at P up to 4 GPa, T up to 1400C, partial melting of H2O-containing peridotite, basalt, as well as peridotite-basalt association with an alkaline-carbonate fluid was experimentally studied as a model of the mantle reservoir with protoliths of the subdued oceanic crust. At partial melting of H2O-containing peridotite at P = 3.73.9 GPa, T = 10001300C, critical ratios were observed in the whole studied interval P and T. At partial melting of H2O-containing basalt critical relationships between the silicate melt and the aqueous fluid were observed at T = 1000C, P = 3.7 GPa. At T = 1100C, Na-alkaline silicate melt coexisted with garnetite, at T = 1150 and 1300C with clinopyroxenite. Signs of critical relationships between the carbonated silicate melt and the fluid were observed in peridotite-basalt-alkaline-water-carbonate fluid system at Р = 4 GPa, T = 1400C. The reaction ratios among the minerals of peritotite restite with the substitutions of Ol Opx Ca-Cpx K-Amf indicated a high chemical activity of the supercritical fluid melt. The results of the experiments suggest that in the fluid-containing upper mantle with supercritical Р-Т there are areas of partial melting (asthenosphere lenses), containing near-solidus supercritical fluid-melts enriched with incompatible elements, with high reactivity. Mantle reservoirs with supercritical fluid-melts, similar in geochemical terms to the enriched mantle, can serve as a source of magma enriched with incompatible elements. The modal and latent metasomatism of the upper mantle under the influence of supercritical fluid-melts leads to the peridotite refertilization due to the enrichment of restite minerals with incompatible elements and its eclogitization.